Solid carbonaceous formations such as coal which are found at the subterranean locations in the earth's crust often contain substantial amounts of light hydrocarbons, primarily low molecular weight hydrocarbon gases, principally, methane. In addition to methane, other fluids such as nitrogen, carbon dioxide and small amounts of heavier gaseous hydrocarbons, principally ethane, can be found in the carbonaceous matrix of the coal or other carbonaceous material. Although such gases are found in hydrocarbonaceous shales and other hydrocarbonaceous formations, the principal source of these gases is coal beds. Such gases are generally referred to as “coal bed methane,” and although other gases are present as indicated above, they are present in only minor amounts. Typically, coal bed methane will contain at least 90 to 95% volume percent methane and, of course, these gases may take the form of substantially pure methane.
The subterranean coal beds that are often found at depths ranging from 200 to 4,000 feet or deeper and usually include naturally occurring fissures and fractures extending through the solid coal, which provide for the flow of gas within the coal formation. These naturally occurring fractures in coal formations are predominantly vertical fractures which are closely spaced with intervals of perhaps 10 to 100 feet between adjacent fractures. The predominant vertically oriented fractures are referred to as face cleats. Less extensive fractures, which are generally at right angles to the face cleats, are referred to as butt cleats. In some instances, however, fractures present in subterranean coal beds result from fracturing processes such as hydraulic fracturing, employing fracturing fluids and proppants of the type which are well-known in the petroleum industry for the fracturing of subterranean oil reservoirs. Methane in coal beds is found as an absorbed gas in the coal matrix, which is typically in the form of a dense, near-liquid fluid, as contrasted with the free gas found in the gas cap of a petroleum reservoir or dissolved in the liquid petroleum found in petroleum reservoirs. While a small fraction of methane gas may be found in the cleat structure as free gas, a great preponderance of the methane is physically absorbed on the coal structure. The methane is depleted from a coal bed following a desorption isotherm in which the storage capacity of the coal for the methane changes only slightly with reduction in pressure at high pressures, and then changes more rapidly as the pressure in the coal seam is reduced substantially.
Methane is recovered from the subterranean coal bed or other carbonaceous formation by a production system comprising one or more wells extending from the surface of the earth into the subterranean formation. By establishing a pressure gradient from the formation to the well-bore and then to the surface, the methane gas entrained within the coal bed is desorbed and flows through the system of fractures and fissures within the coal bed to the well and is then produced to the surface and transmitted to a suitable methane recovery facility.